A mold such as a mold for plastic molding is practically formed with a tool steel (quenching and tempering steel, HRC of around 50 to 55). For highly efficient cutting work on a mold of this type, a radius end mill that couples end cutting edges and peripheral cutting edges with arc-shaped corner cutting edges has been heavily used. Since the radius end mill has the arc-shaped corner cutting edges, compared with other end mills (for example, a square end mill), the cutting edges involving cutting lengthen. Moreover, the maximum amount of cutout by each cutting edge is decreased. This is advantageous in that a cutting resistance of each cutting edge is small. Consequently, this ensures the improved machining efficiency by, for example, setting a fast feed speed, ensuing a long tool service life.
However, cutting work with the radius end mill lengthens the cutting edges contributing to the cutting; therefore, chatter vibrations are likely to occur. Especially, increasing the feed speed and a notch makes this tendency remarkable. Consequently, in addition to a reduction in the tool service life, this brings a significant problem such as a deterioration of roughness of a processed surface. Therefore, to improve the roughness of the processed surface (a grade of the processed surface), Patent Documents 1 and 2 have proposed the following.
As illustrated in FIG. 6(a) and FIG. 6(b), with the radius end mill in Patent Document 1, a back tapered peripheral cutting edge 111 is smoothly coupled to an outer peripheral side boundary 103a of an arc cutting edge 103, which is disposed at an end cutting edge of a tool body 102. The peripheral cutting edge 111 is formed into an arc from a boundary with the arc cutting edge 103 along a common tangent line L1. The radius end mill includes a face 104 on a front side of each arc cutting edge 103 in a rotation direction. A swarf discharging groove 105 is formed on the front side including the face 104 in the rotation direction. The swarf discharging groove 105 is excised into an approximately V shape in a central axis line O direction in a horizontally cross-sectional view. On the rear side of the arc cutting edge 103 in the rotation direction, a primary flank 107, which has a positive clearance angle, and a secondary flank 108, which has a clearance angle larger than the positive clearance angle, are continuously disposed. At an other (the central axis line O side) boundary 103b on each arc cutting edge 103, a linear-shaped first end cutting edge 112 and a second end cutting edge 113 are disposed consecutively inclined to a base portion side of the tool body 102. The first end cutting edges 112 and the second end cutting edges 113 intersect with one another at a chisel 114, which is disposed on the central axis line O.
According to a radius end mill 101 in Patent Document 1, a radius of the peripheral cutting edge 111 is configured larger than a radius of the arc cutting edge 103 (configured at a dimension twice to ten times as large as a radius of the tool body). The arc cutting edge 103 couples the first end cutting edge 112 at a boundary on the opposite side from the peripheral cutting edge 111 at a clearance angle of 0.5° to 5°. To the first end cutting edge 112, the second end cutting edge 113 is coupled providing a larger clearance angle of 7° to 15°. This leads to smooth cutting at an outer peripheral side boundary 103a between the arc cutting edge 103 and the peripheral cutting edge 111, restraining chatter vibrations and providing good surface roughness. Further, the clearance angle of the first end cutting edge 112 reduces a cutting load on a bottom surface of workpiece, restraining the chatter vibrations and providing good finishing of the processed surface.
A radius end mill in Patent Document 2 is a long neck radius end mill with a cutting diameter of 6 mm or less and an under-neck length three times or more of the cutting diameter. As illustrated in FIG. 7(a) and FIG. 7(b), the cutting part has a plurality of peripheral cutting edges 16, radius end cutting edges 17, and end cutting edges 18. The end cutting edge 18 includes an outer peripheral side end cutting edge 19, which is coupled to the radius end cutting edge 17, and an inner peripheral side end cutting edge 110, which is coupled to the outer peripheral side end cutting edge 19 and heads for a center direction of the tool. With this long neck radius end mill, the radius end cutting edge 17 is smoothly coupled to the outer peripheral side end cutting edge 19. Therefore, the radius end cutting edge 17 is slightly extended to the inner peripheral side and is smoothly coupled such that a tangent line of the radius end cutting edge 17 at a final point of the extension becomes the outer peripheral side end cutting edge 19. Thus, as illustrated in FIG. 7(a), for smooth coupling between the radius end cutting edge 17 and the outer peripheral side end cutting edge 19 at the junction, Patent Document 2 configures a concave angle θ1 of the outer peripheral side end cutting edge 19 and a concave angle θ2 of the inner peripheral side end cutting edge 110 within a specified range.
As illustrated in FIG. 7(b), designing a length Lc of the outer peripheral side end cutting edge 19 within a specified range ensures reliable cutting and removal of an uncut part even in the case of a large feed amount of one cutting edge. Further, the outer peripheral side end cutting edge 19 of the end cutting edge 18 is inclined to a rear side with respect to the inner peripheral side end cutting edge 10 in the rotation direction at a predetermined angle θ3. This restrains the chatter vibrations in the long neck radius end mill, preventing deterioration of roughness of the processed surface.